The present invention relates to a low-viscosity two-component filled polyurethane adhesive comprising an organic polyisocyanate component and an isocyanate-reactive curative component. The curative component comprises a phase-stable blend of a relatively high equivalent weight isocyanate-reactive polyether component with 2-methyl-1,3-propanediol and other optional chain extenders or crosslinkers, including low molecular weight diamines or triamines that impart sag resistance.
Urethane-based adhesives are known for use in joining together various plastic materials. Although certain preformed polyurethanes can be used as adhesives by applying heat (for example, U.S. Pat. No. 4,156,064), the preferred urethane adhesives are typically two-component urethane-based adhesives comprised of an isocyanate component and an isocyanate-reactive curative component. Such adhesives are preferred over other adhesives, at least in part because of outstanding bond strength, flexibility, and resistance to shock and fatigue.
Various approaches for preparing improved two-component urethane adhesives have been described. One approach uses certain special reagents to improve the properties of the adhesives. For example, the addition of certain amide acetals to the curative component improves adhesive properties by inhibiting foaming. E.g., U.S. Pat. No. 4,728,710. The use of certain special isocyanate-reactive compounds (for example, special amines, amino alcohols, and thiols) as part of the curative component also provides improved adhesives. E.g., U.S. Pat. Nos. 3,714,127, 3,812,003, 3,935,051, and 4,336,298.
It is possible to improve adhesive properties without the need for special reagents of the types described above. For example, U.S. Pat. Nos. 3,979,364 and 4,743,672 disclose two-component urethane adhesives in which the curative component contains mixtures of polyols and polyoxyalkylene polyamines. These polyamines, in which the amino groups are bound to aliphatic carbon atoms, impart sag resistance as well as improved adhesion.
Efforts to improve adhesive properties have also focused on the polyol component of the curative component. For example, U.S. Pat. No. 4,336,298 specifically requires the use of polyester or polyether triols having a molecular weight range of about 400 to 1000, which corresponds to an equivalent weight of no more than about 333. U.S. Pat. No. 4,444,976 specifies a curative component containing polyols having at least two hydroxyl groups and a molecular weight range of about 100 to 2000, which corresponds to an equivalent weight of no more than 1000. Higher molecular weight polyols are disclosed but only for the preparation of prepolymers used as the polyisocyanate component. U.S. Pat. No. 4,552,934 discloses a curative component containing hydroxyl terminated prepolymers prepared by the reaction of organic polyisocyanates, polyols having a molecular weight range of about 150 to 3000, and polyamines. European Patent Application 304,083 discloses an isocyanate component containing a blend of an aliphatic isocyanate and an aromatic isocyanate prepolymer and a curative component containing a polyfunctional polyether polyol having a hydroxyl number of from 100 to 1200 (corresponding to equivalent weights of about 45 to about 561), a diamine, and an optional catalyst. Higher molecular weight polyols are disclosed but only for the preparation of the prepolymers of the isocyanate component.
Japanese Patent 89/48,876 discloses a two-component urethane adhesive having a polyisocyanate component and a curative component containing a mixture of relatively high molecular weight polyols, relatively low molecular weight polyols, zeolite filler, and catalyst. The patent, however, does not disclose any special role for 2-methyl-1,3-propanediol.
European Patent Application 328,808 discloses a two-component urethane adhesive having an isocyanate-terminated urethane prepolymer component and a curative component containing a nitrogen-free polyol (preferably a low molecular weight diol) and a primary amine for sag resistance, as well as optional fillers and other additives. In contrast to the present invention, the European application does not suggest the importance of using a curative component containing both relatively high equivalent weight polyether polyols and 2-methyl-1,3-propanediol or other low molecular weight diol chain extenders or crosslinkers.
The use of low viscosity components in two-component adhesive systems is desirable as long as sag is not excessive. U.S. Pat. No. 4,552,934 describes the desirability of low viscosity components having viscosities of 35,000 cps (i.e., 35,000 mPa.s) for the isocyanate component and approximately 50,000 cps for the curative component. Although described as having low viscosities, both adhesive components of the patent are considerably more viscous than those of the present invention. U.S. Pat. No. 4,336,298 discloses a low viscosity hardener component having a viscosity of from 400 to 8000 mPa.s but requires a higher viscosity isocyanate component having a viscosity of from 20,000 to 55,000 mPa.s.
Two-component adhesives based on isocyanate-reactive curative blends containing relatively high equivalent weight isocyanate-reactive polyethers, low molecular weight diol chain extenders or crosslinkers, and amine have been disclosed. E.g., U.S. Pat. Nos. 4,994,540, 5,164,473, and 5,204,439. However, the curative components of such adhesives have typically been unstable with respect to separation into phases.
It has now surprisingly been found that isocyanate-reactive curative blends containing a relatively high equivalent weight polyether component, 2-methyl-1,3-propanediol, and optional chain extenders and/or crosslinkers other than 2-methyl-1,3-propanediol are stable to phase separation while still exhibiting low viscosity. Such blends, when used with a low viscosity polyisocyanate component, provide two-component adhesives having advantageous properties. For example, adhesives according to the invention, although used without primer, exhibit excellent high temperature bonding strength. When low molecular weight isocyanate-reactive diamines or triamines am used as chain extenders or crosslinkers, the mixed adhesives of the invention exhibit excellent resistance to flow, or "sag", despite the fact that both reactive components are characterized by low viscosities of less than about 15,000 mPa.s.
Although some of the compositions described as useful in the above references can also be used with the present invention, none of the references discloses or suggests the combinations of components that are critical to this invention. In particular, none discloses the use of a curative component containing mixtures of a relatively high equivalent weight isocyanate-reactive polyether and 2-methyl-1,3-propanediol (and other optional low equivalent weight chain extenders or crosslinkers) that are stable to phase separation.